1) Field of the Invention
The present invention relates generally to obtaining synchronization between two telecommunication devices, and more specifically to recognizing in an incoming bit stream, a framing pattern of M single bits distributed at constant intervals of a fixed number N, of bits, as measured from the start of one framing bit to the start of the next.
2) Prior Art
In telecommunication industry, communication between devices consists quite often in two separate serial bit streams in opposite directions, each device being the `emitter` for one serial bit stream, and the `receiver` for a second stream. Some sort of synchronization convention is necessary between the emitter and the receiver so that they can `talk` to each other. For that purpose, a technique widely used consists in the emitting device repeatingly embedding in the transmitted bit stream, a preassigned M-bit framing pattern that is distributed at constant intervals of a fixed number, N, of bits, as measured from the start of one framing bit to the start of the next, and the receiving device recognizing the M-bit pattern in the bit stream thus allowing synchronization so that the rest of the bit stream can make sense.
This technique is for example used in CCITT recommendation X.50 in an effort to standardize a preferred multiplexing scheme to be used on links between different countries. In that case, data transmission is achieved at 64 kbps, the bit stream being a sequence of 8-bit envelopes as follows: EQU ...DDSFDDDDDDSFDDDDDDSFDDDDDDSFDDDDDDSFDDDDDD....
where
D is the data transmitted PA1 S is a status bit PA1 F is a framing bit part of the 20-bit framing pattern being: A1101001000010101110 PA1 D is the data transmitted PA1 S is a status bit PA1 T is: 10H10H10H10H... (H: housekeeping bit) PA1 F is a framing bit part of the the 20-bit framing pattern as above
(A is a signalling bit)
This technique is also used in Italian CDN network as defined in `Capitolato tecnico Nr.1222 Apparecchiature DCE` and `Capitolato tecnico Nr.1267 Apparecchiature DCE-2` from SIP (Societa Italiana per l`Esercizio delle Telecomunicazioni`). In that case, data transmission is achieved at 72 kbps, the bit stream being a sequence of 9-bit envelopes as follows: EQU ...TFDDDDDDSTFDDDDDDSTFDDDDDDSTFDDDDDDSTFDDD....
where
Finally this technique is used to a certain extent for multiframing purpose on the ISDN Basic Rate Interface (192 kbps).
Therefore it is of major importance, for the man skilled in the art, that a system achieving fast and secure synchronisation on such a distributed framing pattern (pattern of M bits embedded in the serial data stream at intervals of N bits) be implemented in telecommunication products. A prerequisite to that synchronization is a fast and secure recognition of the framing pattern.
U.S. Pat. No. 4,611,336 to Fryer describes a method and associated system for frame synchronisation for serial bit streams having an embedded framing pattern. The method consists in extracting from the bit stream, consecutive groups of N bits, incrementing or resetting N associated counters depending on the coincidence or not of each one of the N bits with an expected bit of the framing pattern. When one of the N counters reaches M, recognition of the pattern is said to be achieved, thus synchronization made possible.
European Patent Application 0 161 177 to Bojarski also describes a method and associated system for achieving synchronisation on serial bit stream. But method consists in extracting M bits from the bit stream, said M bits incoming at intervals of N bits, and comparing the M-bit word thus obtained with the framing pattern or its (M-1) rotations. Synchronisation is said to be achieved if comparison is true logically This system requires quite a bit of fast hardware depending on the speed of the incoming data stream.
Both methods have the main following drawbacks:
Poor security: there is always a chance for a `look-alike` M-bit pattern to be detected in the data portion of the bit stream and the synchronization flagged as being achieved when it is actually not (resulting in a time-consuming recovery process after a fake synchronization). To avoid this, double or triple checking is made necessary (counter reaching a multiple of M in U.S. Pat. No. 4,611,336 to Fryer; once pattern is detected, having to confirm it with a new search in European Patent Application 0 161 177 to Bojarski). PA0 Low speed of synchronisation: this speed is dependant probability of `taking a good start` in the pattern search.